Fixing a broken volume knob?

So my volume knob broke off while I was transporting my bass amp (a Fender Bassman 60) in my car yesterday and I can no longer snap the knob back into place because the shaft that sticks out of the amp (this is what the knob is usually attached to) seems to have come loose and falls back into the amp whenever I try to snap the knob back on.

Who else has dealt with this issue and did you fix it yourself or take it into a music shop for repair? If you took in in for repair, how much did it cost? I need to get the amp in working form by next Sunday so any knowledgeable responses would be much appreciated.

The collar of the volume pot is threaded, and there should be a nut that holds the volume pot tight against the control plate. If the threads are stripped, you'll probably have to replace the pot - should be minimum bench-charge at your local repair shop.